X-rays and у photons give up their energy in three different ways, and the way in which they do it depends on the energy of the photon. One way is called the pho­toelectric interaction—this is precisely the interaction that Einstein explained and for which he received the Nobel Prize. A second way is known as the Compton interaction, named for Arthur Compton, the American scientist who discovered it, who also received a Nobel Prize for his discovery. The third way is called pair production, which involves the creation of positive and negative electrons (2).

Photoelectric Interaction

The photoelectric interaction involves a collision of an energetic photon with an inner-shell electron of an atom, as in the Bohr model of the atom (Figure 7.1). All of the energy of the photon is given to the electron, which is kicked completely out

of the atom with the energy of the photon minus the energy that bound it to the nucleus, and the photon disappears. This electron can then cause damage on its own by charged particle interactions. Low energy X-rays are also given off when an electron jumps from a higher shell to fill the vacancy left by the electron kicked out of the atom.

The photoelectric interaction occurs primarily for low energy photons with energies below about 100 kiloelectron volts (keV)1 and its cross section—the prob­ability of an interaction—goes up rapidly with the atomic number (Z) of the atom it interacts with. Mathematically speaking, the cross section is proportional to Z4/ E3, where E is the energy of the photon. This has very important practical applica­tions in diagnostic radiology, the science of using radiation to image bones and other body parts. Our tissues other than bone mostly consist of carbon (Z = 6), nitrogen (Z = 7), oxygen (Z = 8), and hydrogen (Z = 1). Bone, however, has a large amount of calcium with a Z of 20. Low energy photons such as X-rays with ener­gies of about 75 keV will primarily interact by the photoelectric effect, and bone will absorb photons a lot better than the soft tissue. The reason an X-radiograph shows the bones so well is because they absorb most of the photons, leaving a dark image, compared to the soft tissue, which doesn’t absorb the photons very well. The photoelectric interaction is also the basis for using contrast solutions to image body parts such as the colon. A solution of barium (Z = 56) is drunk and, as it flows through the digestive system, low energy X-rays can image its location and determine if there are blockages.